1. Field of the Invention
The present invention relates to a method for producing a silicon single crystal having few crystal defects, as well as to a silicon single crystal wafer produced by the method.
2. Description of the Related Art
Along with a decrease-in size of semiconductor devices for achieving an increased degree of integration of semiconductor circuits, more severe quality requirements have recently been imposed on silicon single crystals which are grown by the Czochralski method (hereinafter referred to as the CZ method) for use as materials for substrates of semiconductor circuits. Particularly, there has been required a reduction in density and size of grown-in defects such as flow pattern defects (FPDs), laser scattering tomography defects (LSTDs), and crystal originated particles (COPs), which are generated during the growth of a single crystal and degrade oxide dielectric breakdown voltage and characteristics of devices.
In connection with the above-mentioned defects incorporated into a silicon single crystal, first are described factors which determine the concentration of a point defect called a vacancy (hereinafter may be referred to as V) and the concentration of a point defect called an interstitial-Si (hereinafter may be referred to as I).
In a silicon single crystal, a V region refers to a region which contains a relatively large number of vacancies, i.e., depressions, pits, voids or the like caused by missing silicon atoms; and an I region refers to a region which contains a relatively large number of dislocations caused by excess silicon atoms or a relatively large number of clusters of excess silicon atoms. Further, between the V region and the I region there exists a neutral (hereinafter may be referred to as N) region which contains no or few excess or missing silicon atoms. Recent studies have revealed that the above-mentioned grown-in defects such as FPDs, LSTDs, and COPs are generated only when vacancies and/or interstitials are present in a supersaturated state and that even when some atoms deviate from their ideal positions, they do not appear as a defect so long as vacancies and/or interstitials do not exceed the saturation level.
It has been confirmed that the concentration of vacancies and/or interstitials depends on the relation between the pulling rate (growth rate) of a crystal in the CZ method and the temperature gradient G in the vicinity of a solid-liquid interface of the crystal, and that another defect called oxidation-induced stacking fault (OSF) is present in ring-shape distribution in the vicinity of the boundary between the V region and the I region.
The manner of generation of defects due to growth of a crystal changes depending on the growth rate. That is, when the growth rate is relatively high; e.g., about 0.6 mm/min, grown-in defects such as FPDs, LSTDs, and COPsxe2x80x94which are believed to be generated due to voids at which vacancy-type points defects aggregatexe2x80x94are present at a high density over the entire radial cross section of a crystal. The region where these defects are present is called a xe2x80x9cV-rich regionxe2x80x9d (see FIG. 5(a)). When the growth rate is not greater than 0.6 mm/min, as the growth rate decreases the above-described OSF ring is produced from a circumferential portion of the crystal. In such a case, L/D (large dislocation, simplified expression of interstitial dislocation loop) defects such as LSEPDs and LFPDsxe2x80x94which are believed to be generated due to dislocation loopxe2x80x94are present at a low density outside the OSF ring. The region where these defects are present is called an xe2x80x9cI-rich regionxe2x80x9d (see FIG. 5(b)). Further, when the growth rate is decreased to about 0.4 mm/min, the above-described OSF ring converges to the center of a wafer and disappears, so that the I-rich region spreads over the entire cross section of the wafer (see FIG. 5(c)).
Further, there has been found the existence of a region, called an N (neutral) region, which is located between the V-rich region and the I-rich region and outside the OSF ring and in which there exist neither grown-in defects (FPDs, LSTDs, and COPs) stemming from voids nor L/D defects (LSEPDs and LFPDs) stemming from a dislocation loop (see Japanese Patent Application Laid-Open (kokai) No. 8-330316). The N region has been reported to be located outside the OSF ring and is located on an I-Si side, so that substantially no oxygen precipitation occurs when a single crystal is subjected to a heat treatment for oxygen precipitation and the contrast due to oxide precipitates is observed through use of an X-ray beam. Further, the N-region has been reported to be not rich enough to cause formation of LSEPDs and LFPDs (see FIG. 4(a)). It has been proposed that the N region can be expanded over the entire wafer surface when a ratio F/G is controlled to fall within the range of 0.20-0.22 mm2/xc2x0C.xc2x7min through an improvement of the intra-furnace temperature distribution and adjustment of the pulling rate, wherein F is a pulling rate (mm/min) of the single crystal, and G is an average intra-crystal temperature gradient (xc2x0C./mm) within a temperature range of the melting point of silicon to 1300xc2x0 C. (see FIG. 4(b))
However, when a single crystal is produced such that the region having a very low defect density is expanded to the entire crystal, the control range of production conditions becomes extremely narrow, because the region must be an I-Si side N region. Setting aside experimental apparatus, such precise control is difficult to perform in a mass-production-type apparatus. Further, since productivity is low, the proposed technique is not practical.
Further, the inventors of the present invention found that the defect distribution chart shown the in above-mentioned patent publication greatly differs from data that the inventors of the present invention obtained through experiments and investigations and consequently from a defect distribution chart (see FIG. 1) that was made based on the thus-obtained data.
Further, the N region distributed outside the OSF ring was found to include a region where a larger amount of precipitated oxygen (hereinafter may be referred to as an xe2x80x9cN2(V)xe2x80x9d region), and a region where a smaller amount of precipitated oxygen (hereinafter may be referred to as an xe2x80x9cN(I)xe2x80x9d region). Therefore, if a wafer is merely produced in the N region outside the OSF ring, the N2(V) region where a larger amount of precipitated oxygen and the region N(I) where a smaller amount of precipitated oxygen are formed mixedly within the wafer, with the result that the device yield decreases due to a difference in gettering capability.
In view of the foregoing, an object of the invention is to enable highly efficient production of a silicon single crystal in accordance with the CZ method, under production conditions that broaden the range of control and facilitate control, such that the silicon single crystal has neither a V-rich region nor an I-rich region and therefore has an extremely low defect density over the entire cross section of the crystal, as well as a gettering capability stemming from oxygen precipitation.
In order to achieve the above-described object, the present invention provides a method for producing a silicon single crystal in accordance with the CZ method, wherein the single crystal is grown in an N2(V) region where a large amount of precipitated oxygen and which is located within an N region located outside an OSF ring region in a defect distribution chart (see FIG. 1) which shows a defect distribution in which the horizontal axis represents a radial distance D (mm) from the center of the crystal and the vertical axis represents a value of F/G (mm2/xc2x0C.xc2x7min), where F is a pulling rate (mm/min) of the single crystal, and G is an average intra-crystal temperature gradient (xc2x0C./mm) along a pulling direction within a temperature range of the melting point of silicon to 1400xc2x0 C.
In a silicon single crystal wafer produced in accordance with the method of the present invention, neither FPDs nor L/D defects (LSEPDs, LFPDs) exist on the wafer surface, and as shown in FIG. 2(b), neither a V-rich region nor an I-rich region is present on the wafer surface. Instead, the wafer surface is formed of only a N2(V) region which is neutral and in which the amount of precipitated oxygen (xcex94Oi) is large and therefore a high gettering capability is provided.
In this case, as shown in the defect distribution chart shown in FIG. 1, the pulling of a single crystal must be performed within the N2(V) region that is narrow and steeply inclines from the center toward the outer circumferential of the crystal. Therefore, it is difficult to control the pulling conditions such that the same region is used over the entire cross section of the crystal. However, there can be obtained a high-quality and substantially defect-free single crystal wafer in which neither FPDs nor L/D defects (LSEPDs, LFPDs); i.e., neither a V-rich region nor an I-rich region is present on the wafer surface, and which is formed of only a N2(V) region which is neutral and in which the amount of precipitated oxygen (xcex94Oi) is large and therefore a high and uniform gettering capability is provided. Accordingly, the device yield can be greatly increased.
The present invention also provides a method for producing a silicon single crystal in accordance with the CZ method, wherein the single crystal is grown in a region that comprises an N1(V) region located inside an OSF ring region and an N2(V) region of an N region located outside the OSF ring region in the above-described defect distribution chart.
Preferably, the growth of the single crystal is performed such that the above-described F/G value becomes 0.119-0.142 mm2/xc2x0C.xc2x7min at the center of the crystal.
As shown in FIG. 3(b), in a silicon single crystal wafer produced in the above-described manner, there are present an OSF ring or nuclei of the OSF ring that appear in the shape of a ring when the wafer is subjected to thermal oxidation treatment. Further, neither FPDs nor L/D defects exist on the wafer surface, and gettering capability stemming from oxygen precipitation is provided over the entire wafer surface.
As described above, when the single crystal is grown in a region that comprises an N1(V) region located inside an OSF ring region and an N2(V) region of an N region located outside the OSF ring region, as shown in the defect distribution chart of FIG. 1, the single crystal includes a region where an OSF ring may be generated upon performance of thermal oxidation treatment. However, since the single crystal is pulled such that the areas of the N1(V) region and N2(V) region located inside and outside the OSF ring region are maximized, the range of control in relation to the pulling rate and the intra-crystal temperature gradient increases. Therefore, even in a pulling apparatus designed for mass production, production conditions can be easily set, and thus wafers having a large N(V) region can be easily produced.
The present invention also provide a silicon-single crystal wafer grown in accordance with the CZ method, wherein the oxygen concentration of the entire wafer surface is less than 24 ppma (value of ASTM ""79); latent nuclei of an OSF ring are present after heat treatment for oxygen precipitation but no OSF ring is generated when the wafer is subjected to an OSF thermal oxidation treatment; neither FPDs nor L/D defects exist on the wafer surface; and gettering capability stemming from oxygen precipitation is provided over the entire wafer surface.
When the production method of the present invention is performed, the pulling of the single crystal is preferably controlled such that the time required for passing through a temperature zone of 1050xc2x0 C.-850xc2x0 C. within the crystal becomes 140 minutes or less.
When the oxygen concentration within a single crystal being grown is suppressed to less than 24 ppma or the thermal history of the single crystal is controlled such that the time required for passing through a temperature zone of 1050xc2x0 C.-850xc2x0 C. within the crystal becomes 140 minutes or less, growth of an OSF nucleus can be prevented. Therefore, even when an OSF ring or latent nuclei of the OSF ring exist in a wafer, devices are not affected. That is, although latent nuclei of an OSF ring are present in a wafer when the wafer is subjected to an OSF thermal oxidation treatment, no OSF ring is actually generated. Thus, there can be obtained a wafer in which neither FPDs nor L/D defects (LSEPD, LFPD) are generated on the wafer surface; i.e., none of a V-rich region, an I-rich region, and a harmful OSF ring exists on the wafer surface; whose entire surface therefore can be used; and which has an extremely low defect density over the entire wafer surface and can provide over the entire surface gettering capability stemming from oxygen precipitation. In addition, since the single crystal is pulled such that the areas of the N1(V) region and N2(V) region located inside and outside the OSF ring region are maximized, the range of control of F/G can be widened, and thus production of wafers can be facilitated.
As described above, the present invention enables production of a wafer whose entire surface is occupied by an N2(V) region and which can provide gettering capability. Especially, since an N2(V) region outside the OSF ring, an N1(V) region inside the OSF ring, and an OSF ring or nuclei of the OSF ring are used, the range of control on the conditions of growth of a single crystal becomes wider, so that there are produced wafers in which the area of the N(V) region is maximized. When oxygen concentration reduction or thermal history control in a low temperature zone is additionally employed, no OSF ring is generated, so that it becomes possible to produce a uniform silicon single crystal wafer which has a very low grown-in defect density, whose entire surface is substantially free of defects, and which can provide over the entire wafer surface enhanced gettering capability stemming from oxygen precipitation.